1. Field of the Invention
The present invention relates to a flexible board bent at predetermined positions and an electronic device including the same.
2. Description of the Related Art
A conventional flexible board of this type is described in, for example, International Publication No. WO2012/073591. As shown in FIG. 11, the flexible board 50 described in International Publication No. WO2012/073591 includes a flexible base 501 and a plurality of external terminals 503.
The base 501 includes a plurality of dielectric sheets. Each dielectric sheet is made of flexible, thermoplastic resin such as polyimide or liquid crystal polymer. Moreover, the dielectric sheet, when viewed in a plan view in a direction z (referred to below as a normal direction z) parallel to a line normal to its principal surface, is in the shape of a rectangle extending in a direction x perpendicular to the normal direction z. The base 501 is formed by laminating the dielectric sheets in the normal direction z. In addition, signal lines and ground conductors are provided in or on the base 501 during the course of lamination.
The external terminals 503 are rectangular conductors positioned at opposite ends of the base 501 in the perpendicular direction x, and used as, for example, signal terminals or ground terminals. These conductors are made of a metal material, preferably, a metal foil, mainly composed of silver or copper and including a low specific resistance.
The flexible board 50 is used for electrically connecting two high-frequency circuits within the housing of an electronic device by the external terminals 503. In the housings of electronic devices of recent years, various components and modules are integrated with high density. To place the flexible board 50 in such a housing, the flexible board 50 is bent in accordance with the shape of the space in which it is to be placed. In the example of FIG. 11, the flexible board 50 is bent at four portions A to D, resulting in a first sheet section 505a, second sheet sections 507a and 507b, and bent sheet sections 509a and 509b. 
The first sheet section 505a is a center portion of the flexible board 50, and is positioned at a different level from the second sheet sections 507a and 507b in the normal direction z. A first end of the first sheet section 505a in the perpendicular direction x (namely, bend B) is connected to a first end of the bent sheet section 509a. In addition, a second end of the first sheet section 505a (namely, bend C) is connected to a first end of the bent sheet section 509b. 
Furthermore, the second sheet section 507a is positioned on the negative side of the flexible board 50 in the perpendicular direction x, and is typically fixed within the housing. The second sheet section 507a has at least one of the external terminals 503 provided at its first end in the perpendicular direction x, and is connected at a second end (namely, bend A) to a second end of the bent sheet section 509a. 
Furthermore, the second sheet section 507b is positioned on the positive side of the flexible board 50 in the perpendicular direction x, and is typically fixed within the housing. The second sheet section 507b has at least one of the external terminals 503 provided at its first end in the perpendicular direction x, and is connected at a second end (namely, bend D) to a second end of the bent sheet section 509b. 
The bent sheet section 509a is connected between the first sheet section 505a and the second sheet section 507a so as to be perpendicular thereto. The bent sheet section 509b is connected between the first sheet section 505a and the second sheet section 507b so as to be perpendicular thereto.
However, depending on the bending manner, high-frequency characteristics (in particular, isolation) might vary between individual flexible boards 50 housed in a plurality of electronic devices. For example, the bent sheet sections 509a and 509b are flexible and extend in the normal direction z, and further, the first sheet section 505a is connected on top of both of them. Accordingly, as the size of the first sheet section 505a increases, the shape of the bent sheet sections 509a and 509b becomes unstable. As a result, for example, the signal lines in the bent sheet section 509a might be positioned closer to the signal lines in another sheet section or an external high-frequency circuit, causing crosstalk between them. In addition, the first sheet section 505a might be deflected, for example, under its own weight, causing crosstalk.